本研究旨在探討足以影響費慈定理之預測結果的三個中介變項，此即：肌肉特性、肢體靈活度、以及設備重量。研究者抽取46位國立台東大學特殊教育系學生進行三項實驗。其結果如下： 1.肌肉特性與肢體靈活度可視為影響費慈定理之預測結果的中介變項。 2.就肌肉特性而言，操作輔具之動作特性應配合其所需之肌肉特性，亦即：屬於大（小）肌肉之操作動作宜以相對應之大（小）肌肉為之。 3.就肢體靈活度而言，使用者之肢體障礙狀態可影響其操作輔具之表現，此突顯生產輔具之通用設計概念的重要性。 4.肌肉特性與肢體靈活度並非為影響打字錯誤之主要因素，依據受試者之見解：其主要因素應為受試者之鍵盤熟悉度。 5.本研究結果並無法支持輔具之設備重量可視為費慈定理之中介變項。

This study aimed to examine three intervening variables (i.e., muscular trait, limb mobilization, and device weight) which were supposed to have influence on the prediction of Fitts’ law. The researcher sampled 46 college students from the Special Education Department of National Taitung University and treated them with three experiments. The results manifested as follows: 1. Muscular trait and limb mobilization could be viewed as the intervening variables which affected the prediction of Fitts’ law. 2. For muscular trait, the body action of operating assistive aids should fit its demanded muscular trait, namely, the operation in need of gross (fine) muscles should better be performed by gross (fine) muscles. 3. As to limb mobilization, the users’ conditions of physical disability could affect their own performances of operating assistive aids, thus emphasized the concept of universal design in the production of assistive devices. 4. Muscular trait and limb mobilization were both not the major factors of typewriting errors. For the subjects’ opinions, the major factor affecting the typewriting errors was supposed to be the subjects’ keyboard familiarity. 5. Device weight, as the data shown in this study, revealed not robust enough to be an intervening variable for Fitts’ law.